The present invention relates to an air-fuel ratio control apparatus of an internal combustion engine and more particularly to an air-fuel ratio control apparatus having a closed feedback loop.
Generally, an internal combustion engine with a carburetor is equipped with a main fuel supply system for supplying fuel to a cylinder via a main discharge nozzle disposed on the venturi in an intake passage when a throttle valve is opened, and with a slow fuel supply system for supplying fuel to the cylinder via an idle port and/or a slow port in the intake passage when the throttle valve is closed or nearly closed. In such an internal combustion engine, a conventional air-fuel ratio control apparatus controls the amount of fuel to be supplied from both the main fuel supply system and the slow fuel supply system by using the same manipulated variable corresponding to a change in a detected signal from an air-fuel ratio sensor which is, for example, an oxygen concentration sensor disposed in the exhaust system for detecting the concentration of the oxygen component in the exhaust gas.
However, in the case where the air-fuel ratio condition of the engine is controlled according to conventional techniques, the air-fuel ratio of the air fuel mixture fed into the cylinder is suddenly changed in response to a change in the feedback control signal which in turn causes a surging phenomenon of the engine to occur. As a result, according to the conventional techniques for controlling the air-fuel ratio, the driving condition of the engine becomes extremely jerky.
The cause of the occurrence of the above-mentioned undesirable phenomenon is described hereinafter. In both the main fuel supply system and the slow fuel supply system, the amount of fuel to be fed into the cylinder is controlled in the same proportion in accordance with the feedback control signal from the air-fuel ratio sensor. Therefore, the amount of fuel fed from the respective system into the cylinder changes in the same proportion, corresponding to the change in the feedback control signal. Since the fuel from the main fuel supply system is discharged into the intake passage upstream of the throttle valve via the main discharge nozzle, the discharged fuel collides with the surface of the throttle valve to form a liquid flow, and the fuel is thereafter changed into minute particles of fuel at the edge of the end of the throttle valve. Therefore, in the intake passage downsteam of the throttle valve, the variation in the amount of fuel fed from the main fuel supply system per unit time, namely, the variation of the air-fuel ratio per unit time, caused by the fuel fed from the main fuel supply system, is extremely small. However, since the fuel from the slow fuel supply system is directly discharged into the intake passage downstream of the throttle valve via the idle port and/or the slow port, the variation in the amount of fuel fed from the slow fuel supply system per unit time, in other words, the variation in the air-fuel ratio per unit time, caused by the fuel fed from the slow fuel supply system is very large. As a result, a surging operation corresponding to the change in the air-fuel ratio feedback control signal may occur.